Pump generated bias for parametric amplifiers

ABSTRACT

Parametric amplifier varactor bias, generated by the rectification of pump power, varies with pump power level to prevent detuning of the varactor circuit caused by changes in pump power level.

United States Patent [191 [111 3,824,482 Whelehan, Jr. July 16, 1974PUMP GENERATED BIAS FOR [56] References Cited PARAMETRIC AMPLIFIERSUNITED STATES PATENTS [75] Inventor: James J. Whelehan, Jr., Smithtown,3,388,263 6/1968 Daniel 330/49 Primary Examiner-Herman Karl Saalbach[73] Asslgnee' g g Mlwaukee Assistant Examiner-Darwin R. HostetterAttorney, Agent, or Firm-Henry Huff [22] Filed: July 30, 1973 21 Appl.No.: 383,573 E571 ABSTRACT Parametric amplifier varactor bias, generatedby the rectification of pump power, varies with pump power 8 330/ 1 33?level to prevent tletuning of the varactor circuit [58] Field ($53M f556M313, 127, 199 Caused by changes pump 3 Claims, 6 Drawing Figures RFSIGNAL INPUT 52:22 3? 1 RF PUMP L PARAMETRIC SOURCE CIRCUIT AMPLIFIERC'RCULATOR gaQVIBLTAGE f 57 RIF P RTIONAL T0 PUMP POWER DIVIDER-VARACT0R SIGNAL I NETWORK BIAS OUTPUT PAIENIEIIIIIIIBN I 3.824.482

SHIiEI 1 0f 2 RF SIGNAL INPUT 27 PUMP? 7 I RF PUMP SIGNAL PARAMETRIC 3IR ULAT SOURCE CIRCUIT AMPLIFIER C c 0R N do VOLTAGE 5 I l PROPORTIONALRF TO PUMP POWER DIVIDER WARACTOR SIGNAL NETWORK BIAS OUTPUT F/GU/PEAIEIIIEIIIIIIIBAII 3.824.482

SHEET 8 [If 2 DIODE w 29 CAPACITANCE 30 m 32 2a (REVERSE BIAS FIGURE 4SIGNAL II LEVEL-\- *FREQUENCY FIGURE 5 SIGNAL A LEVEL\ 33 36 qFREQUENCYFIGURE 6 PUMP GENERATED BIAS FOR PARAMETRIC AMPLIFIERS BACKGROUND 1.Field The invention pertains to improvements in the generation ofparametric amplifier varactor bias.

2. Prior Art Varactor bias in parametric amplifiers is conventionallysupplied by a separate stable power supply dedicated solely to thispurpose. Variations in pump power are ordinarily minimized by a varietyof methods ranging from feedback leveling loops .to temperaturecontrolled environmental chambers. Although satisfactory compensationhas been obtained with prior systems, it has often been achieved at'theexpense of size, weight, primary power and cost.

SUMMARY The object of this invention is to provide a simple varactorbias supply for a parametric-amplifier and to automatically vary thebias voltage to prevent detuning of the parametric amplifier caused bypump power variations.

According to this invention, a dc voltage generated by rectification ofpump power is used to provide the bias for the varactor. This voltagecan beobtained from a diode connected to .the pumpsource. When the pumpcontains afrequency multipler, this dc voltage is readily obtained bysampling the dc voltage developed across a multipler diode. This dcvoltage, whether obtained from a multipler diode or a separate detectiondiode, varies as a function of pump power, and when adjusted in leveland applied to the varactor as bias, it varies to oppose the variationin varactor capacitance which would occur due to changes in pump powerlevel. This bias thereby maintains a constant average varactorcapacitance and prevents the detuning of the parametric amplifier.

DRAWINGS FIG. 4 is a graph of .a typical varactor capacitancecharacteristic as a function of reverse bias.

FIG. 5 is a graph of the passband characteristic of a typical doubletuned parametric amplifier. FIG. 6 is a graph of the passbandcharacteristic of a double tuned parametric amplifier showing the typeof detuning which can occur with a variation in pump power level.

DESCRIPTION Referring to FIG. .1, a source 1 produces RF energy which isapplied to a pump circuit 2. The pump circuit 2 provides a pump signalof desired frequency and power to a parametric amplifier 3. When sourcel produces a signal at the desired pump frequency,the pump circuitmerely connects the source 1 to the parametric amplifier; however, whena pump frequency higher than the source frequency is desired, the pumpcircuit normally includes a'frequency multiplier. The combination of thesource 1 and the pumpcircuit 2 ishereinafter referred to as the pumppower source. The parametric amplifier, when supplied with pump power,develops a negative resistance which is coupledito acirculator 4 toamplify signals passing through the circulator in a known manner. v

A varying dc voltage, used in producingthe varactor bias for theparametric amplifier, is obtained .from a diode connected to pumpcircuit 2. This voltage is applied to the varactor by way of dividernetwork 5. When a frequency multiplier is used in thepump circuit, thevarying dc voltage may be obtained from a frequency multiplier diode.

The parametric amplifier 3 contains a varactor or varactors andresonators arranged in known manner, for example as shown in USE-Pat.No. 3,105,941. The varactor is normally aback biased diode exhibiting anonlinear capacitance characteristic as a function of voltage. Atypicalvaractor characteristic "is illustrated in FIG. 4. The ordinate 27represents diode capacitance while the abscissa 28 represents reversediode bias. Curve 29 represents the varactor capacitance characteristic.Points 30, 31, 32represent various bias points at which 'the varactormay be operated. In normal operation, the 'varactor is biased at one ofthese points so that the pump swing about this "bias pointdoes not drivethe varactor appreciably into the forward bias direction.

The varactor capacitance is instantaneously varied by the voltageswingof the pump signal; however, this varying capacitance appears asa'fixed average capacitance to a resonant circuit when the :pump andbias levels .are constant. When the pump level is not constant, the:a'veragecapacitance of the varactor varies in accorproduce an increasein average varactor capacitance.

The reason for the increase in capacitance with an increase in pumpsignal level can be seen by examining FIG. 4. An increase in swing abouta bias point will increase the peak voltage excursion both to the leftand rightof the bias point. To 'the left, the capacitance increasesrapidly while to the right it drops slowly. The resultantover acompletecycle is an overall'increase in average capacitance. The detuning effectcaused by such an increase in capacitance can be compensated for by anincrease in reverse biasplacing the quiescent "bias point at alowercapacitance level. Such a change in bias would be exhibited in FIG.4 by moving from Bias point 31 to bias :point 32. g

In FIG. '5 and FIG. -6, the ordinate 33 represents amplitude, while theabscissa 34 represents frequency. A typical double tuned parametricamplifier bandpass characteristic 35 is shown in FIG. '5. Anuncompensated variation in pump power level will detune the resenators,distorting the passband characteristic as shown by bandpasscharacteristic 36 in FIG. 6. This figure shows peaking at the high endof the band and corresponds to a reduction in varactor capacitance and areduction in pump power level. Compensation .for this change requires areduction in reverse bias level, which plied to the input terminals 6and 7 of the frequency multiplier is passed through filter 8 to diode10. A portion of the fundamental energy is converted in diode 10 to thedesired harmonic frequency. The harmonic frequency is passed throughfilter 9 and matching trans former 13 to the output terminals 14 and 15.Filter 8 represents a block to the harmonic frequency preventing it fromflowing to the'input and filter 9 represents a block to the fundamentalfrequency preventing it from flowing to the output. A portion of the RFpump power is normally converted to a dc voltage which, in accordancewith this invention, is used to produce the varactor bias voltage. Thedc voltage is obtained from terminals'll and 12 which are connectedacross multiplier diode 10. This dc voltage is isolated from theremainder of the multiplier circuit by capacitors l6 and 17 connected todiode 10. Since parametric amplifiers normally operate at microwavefrequencies, the symbols used'in FIG. 2 should be considered, whenappropriate, as representing the equivalent microwave circuit element.

The diode contained in the frequency multiplier illustrated in FIG. 2not only functions as a frequency multiplying element, but alsofunctions as a shunt rectifier with the rectified output varying inaccordance with the level of pump power. As'previously described, the dcvoltage generated'in the multiplier diode may not be proper in amplitudeto set the correct varactor bias. Therefore, this amplitude level isadjusted as necessary in a divider network. I

A typical-divider network is shown in FIG. 3. This network reduces thebias level through a resistive divider formed by resistors'20, 21 and22. If necessary, temperature compensation may be provided by atemperature compensation element such as diode 26, which is shuntedacross resistors 21 and 22. The temperature compensating diode may beused for the purpose of overall system compensation, adjusting for theeffects of temperature on a number of components including the dividernetwork, the multiplier diode and the varactor- The dc voltage from thedetection or multiplierdiode is accepted by the divider network atterminals 18 and 19 and filtered in a low-pass filter 25 connectedacross these terminals to remove RF components. Filter 25 is a symbolicrepresentation of more complete filtering circuitry, which normallyincludes, in addition to the shunt capacitor, other components such as ahigh impedance series input element. After filtering and division, thebias voltage is delivered at output terminals 23 and 24 connected acrossresistor 22. The voltage developed across these terminals is a varactorbias voltage of the proper amplitude and sense to prevent detuning ofthe parametric amplifier.

I claim:

1. A system for providing dc bias for the varactor o a parametricamplifier which includes a pump power source, wherein the improvementcomprises:

a. an asymmetrically conductive diode connected to said pump powersource for producing a dc voltage which variesas a function of pumppower level, and i b. means for utilizing a proportion of said dcvoltage to vary said bias, said proportion being chosen to substantiallyequalize the opposing effects of variation in dc bias level andvariation in pump power level upon the tuning of the parametricamplifier.

2. A system for providing bias as recited in claim 1, wherein the pumppower source includes a frequency multiplier and said diode is amultiplier diode in said frequency multiplier.

3. A system for providing bias as recited in claim 1, wherein said meansfor utilizing a proportion of said dc voltage includes a voltage dividerfor applying to the varactor a constant proportion of said dc voltage,which approximately equalizes the opposing effects of variationsin dcbias level and variations in pump power level upon the tuning of theparametric amplifier over a range of pump power levels about a nominaldesign level.

1. A system for providing dc bias for the varactor of a parametricamplifier which includes a pump power source, wherein the improvementcomprises: a. an asymmetrically conductive diode connected to said pumppower source for producing a dc voltage which varies as a function ofpump power level, and b. means for utilizing a proportion of said dcvoltage to vary said bias, said proportion being chosen to substantiallyequalize the opposing effects of variation in dc bias level andvariation in pump power level upon the tuning of the parametricamplifier.
 2. A system for providing bias as recited in claim 1, whereinthe pump power source includes a frequency multiplier and said diode isa multiplier diode in said frequency multiplier.
 3. A system forproviding bias as recited in claim 1, wherein said means for utilizing aproportion of said dc voltage includes a voltage divider for applying tothe varactor a constant proportion of said dc voltage, whichapproximately equalizes the opposing effects of variations in dc biaslevel and variations in pump power level upon the tuning of theparametric amplifier over a range of pump power levels about a nominaldesign level.